Since aromatic polyesters typified by polyethylene terephthalate and polyethylene-2,6-naphthalate have excellent mechanical properties, dimensional stability and heat resistance, they are widely used in films. Especially polyethylene-2,6-naphthalate has more excellent mechanical properties, dimensional stability and heat resistance than polyethylene terephthalate and is therefore used in application fields in which the requirements for these properties are very high, for example, base films for high-density magnetic recording media. However, the requirement for dimensional stability is becoming higher and higher in high-density magnetic recording media, and the further improvement of this property is desired.
Patent Documents 1 to 4 propose a polyalkylene-6,6′-(alkylenedioxy)di-2-naphthoate obtained from a diethyl-6,6′-(alkylenedioxy)di-2-naphthoate which is an ester compound of 6,6′-(alkylenedioxy)di-2-naphthoic acid. According to these documents, a crystalline polyethylene-6,6′-(ethylenedioxy)di-2-naphthoate having a melting point of 294° C. is presented.
However, since the polyalkylene-6,6′-(alkylenedioxy)di-2-naphthoate presented by these patent documents has a very high melting point and very high crystallinity, it has the following problems: when it is to be formed into a film, its extrusion becomes nonuniform due to low fluidity in a molten state, and when it is to be stretched after it is extruded, its crystallization proceeds and it is broken by stretching at a high draw ratio.
Patent Document 3 teaches that a magnetic recording flexible disk having a small tracking shift is obtained by setting the maximum temperature expansion coefficient of a film comprising polyethylene-6,6′-(ethylenedioxy)di-2-naphthoate to 10 to 35 (ppm/° C.), the maximum humidity expansion coefficient of the film to 0 to 8 (ppm/% RH), the difference between the maximum and minimum temperature expansion coefficients to 0 to 6.0 (ppm/° C.) and the difference between the maximum and minimum humidity expansion coefficients to 0 to 4.0 (pm/% RH).
However, the requirement for the improvement of recording density in magnetic recording media is now very high and, accordingly, dimensional stability required for the base film cannot be attained not only with polyethylene terephthalate but also with polyethylene-2,6-naphthalate and a film presented by Patent Document 3.
(Patent Document 1) JP-A 60-135428
(Patent Document 2) JP-A 60-221420
(Patent Document 3) JP-A 61-145724
(Patent Document 4) JP-A 6-145323